A possum weighs about as much as a house cat and eats leaves. A forest is enormous. I could not see how one could possibly beat the other, so I went and read what the scientists measured.
A brushtail possum weighs about three kilograms. That is roughly a house cat. It eats leaves, flowers, buds and fruit, mostly at night, and it is not fast or fierce.
A forest is millions of tonnes of wood, standing there for hundreds of years.
So when I kept reading that possums are killing New Zealand forests, my first thought was that somebody was exaggerating. It sounded like saying a mouse could sink a boat. I went to look at what scientists have actually measured, and the answer turned out to be more interesting than a straight yes or no.
Possums did not walk here. They were brought from Australia in the 1800s to start a fur trade. In Australia they live with animals that hunt them and with trees that have evolved alongside them for a very long time. Here, neither of those things was waiting.
The first thing that surprised me is that possums are fussy. They do not nibble evenly across a forest. They pick favourites - trees like rātā, kāmahi, tōtara, fuchsia, and mistletoe - and they go back to the same trees again and again, night after night.
That is the whole trick, and it is why my mouse-and-boat comparison was wrong.
If a thousand possums each ate a few leaves from a thousand different trees, no tree would notice. Instead, a lot of possums repeatedly strip the same favourite tree. A tree can survive losing some leaves. It cannot survive losing most of them, again, every season, for years, because leaves are how a tree makes food.
Scientists have watched this happen and written it down. In Westland, in a forest of southern rātā and kāmahi, researchers recorded possums arriving around 1950 and, by the 1970s, obvious widespread damage to the canopy - the leafy roof of the forest. When possums were controlled in one block and not in another, the difference in the trees was measurable. Take the browsing pressure off, and the canopy slowly thickens up again.
So the honest version of my question is this: no, one possum cannot kill a forest. Many possums, over decades, eating the same species over and over, can change what a forest is made of.
That is worse than it sounds, because a canopy is not decoration. It is shade, shelter, nesting space and food. When the big favourites thin out and die, the seedlings that come up underneath are the ones possums do not like as much. In a hundred years you still have a forest. You have a different forest - one with fewer of the trees that fed the birds.
And possums do not only eat leaves. They have been filmed eating birds' eggs and chicks in nests, and they eat native snails and insects too. For a long time people thought a possum was simply a vegetarian nuisance. Cameras changed that.
I asked myself whether anyone actually knows how many possums there are. The short answer is that nobody knows precisely. Older estimates of tens of millions are still repeated everywhere, but researchers now treat those numbers cautiously and measure local density instead - how many possums in this valley, this year - because that is the figure you can act on.
Which is a useful lesson on its own. A giant number in a headline can be less true than a small number somebody counted properly.
Question for the reader
Here is what I keep turning over. A possum did not choose to come here. It is not doing anything wrong by possum standards. It is eating the food it can reach, feeding its young, and being extremely good at surviving. If it were rare in Australia and thriving here, we might call it a conservation success.
The reason we call it a pest is entirely about where it is standing.
So: is a possum a pest, or is a possum an animal in the wrong place? Does the difference matter, or is it just wording that makes people feel better about what happens next?
Because what happens next is the part adults argue about. Controlling possums means trapping and poisoning them, and those methods are debated seriously - about which places, which methods, what happens to other animals, what it costs, and who decides. People who all want native birds to survive still disagree strongly about the how. If you want to think about that argument properly, you need two things at once: the ecology, and the ethics.
Try this. Write down what you think a forest is for. A home for native species? A place people can walk in? A thing valuable simply because it is old and complicated and ours? Your answer changes what counts as a fair cost.
Then ask a grown-up in your house the same question, without telling them yours first. I would genuinely like to know whether you got the same answer.